Engine exhaust systems may use various injections of a reductant to assist in the reaction of various exhaust emissions. In one example, the reductant may include Diesel Exhaust Fluid (DEF), which may include a urea-based chemical reactant used in selective catalytic reduction (SCR) to reduce emissions of oxides of nitrogen in the exhaust of diesel vehicles. DEF may be stored in a storage vessel, such as a tank, on-board a vehicle.
DEF may comprise a mixture of a reductant, such as urea, and water. The concentration of the reductant in the in the storage tank, and thus the concentration of reductant injected into the exhaust system, must be known in order to effectively reduce emissions. Government regulations require diesel vehicle to be equipped with a sensor to measure the concentration of DEF.
However, if the DEF freezes and then thaws, the fluid will stratify, creating a varied concentration in fluid within the tank. The reductant, typically heavier than water, will have a higher concentration at the bottom of the tank than the top of the tank. This may lead to inaccurate concentration sensor readings, as the sensor will measure local concentration, and the local concentration may not be equal to the bulk fluid concentration. The local concentration will depend on both the positioning of the sensor within the tank and the orientation of the tank. Until the fluid is thoroughly mixed, the concentration sensor readings will be invalid.
The inventors herein have recognized the above problems and issues and have developed methods and systems to at least partially address them. In one example, a method for an exhaust system is provided, comprising adjusting reductant injection responsive to a reductant concentration, the reductant concentration based on concentration sensor readings and vehicle motion. If the reductant freezes, the reductant may stratify, leading to inaccurate concentration sensor readings. Vehicle motion may mix the reductant, thereby ensuring an accurate concentration measurement which may then be used to adjust reductant injection.
In another example, a method for an exhaust system is provided, comprising indicating a concentration of a reductant stored in a reductant tank based on an output of a concentration sensor coupled within the reductant tank responsive to an indication that a local concentration of reductant at the concentration sensor is within a threshold of a bulk concentration of the reductant stored in the reductant tank, and adjusting a reductant injection profile based on the indicated reductant concentration. If the reductant becomes stratified, for example, following a freeze-thaw event, the local concentration at the concentration sensor may not reflect the bulk concentration. When an indication is received that the local concentration is within a threshold of the bulk concentration, accurate measurements of the reductant can be made using a single sensor. In this way, vehicle emissions may be reduced efficiently.
In yet another example, a vehicle exhaust system is provided, comprising a diesel exhaust fluid tank configured to store a diesel exhaust fluid, a diesel exhaust fluid concentration sensor coupled within the diesel exhaust fluid tank, and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: retrieve a recent valid diesel exhaust fluid concentration measurement, determine a likelihood of a diesel exhaust fluid freeze-thaw event occurring since the recent valid diesel exhaust fluid concentration measurement, set a mixing threshold based on the likelihood of the diesel exhaust fluid freeze-thaw event, determine a concentration sensor entry condition based on vehicle operating condition, indicate a concentration of the diesel exhaust fluid based on an output of the diesel exhaust fluid concentration sensor responsive to the concentration sensor entry condition is greater than the mixing threshold, and adjust a reductant injection profile based on the indicated reductant concentration. The concentration sensor entry condition may be based on vehicle operating conditions, such as speed and/or acceleration, indicating a relative amount of vehicle motion and thus inferring an amount of bulk fluid movement. In this way, the diesel exhaust fluid may be accurately quantified following a freeze-thaw event, allowing for an accurate amount of diesel exhaust fluid to be mixed with exhaust air, thus reducing vehicle emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.